Process for coating strip-form substrates

ABSTRACT

A method of coating a strip of viscous solution on a movable substrate by feeding the coating solution through a narrow gap between coating apparatus which includes feed means and the movable substrate under a static pressure differential between the inlet and the outlet of the gap wherein the gap is part of a coating chamber of the coating apparatus. The gap is wider than the wet formed layer after formation. The coating chamber is connected to a vacuum means at the side of the coating from which the substrate is introduced. A negative pressure is effectuated at the inlet side of the coating apparatus. The vacuum means having a vacuum which seals the inlet end of the narrow gap in the coating apparatus by sucking air from the vacuum means through a diaphragm which withdrawal is at a uniform rate of flow.

0 United States Patent 1151 3,663,292 Herzhoff et al. 1 May 16, 1972 54] PROCESS FOR COATING STRIP-FORM 2,681,294 6/1954 Beguin ..117/34 SUBSTRATES 2,952,559 9/1960 Nadeau ..117 34 3,206,323 9/1965 Miller ..1 17/34 [72] Inventors: Peter Herzhoff, L everkusen Hans Grei', 3,227,136 1/1966 Bartlett- "1 18/63 X cologne-stammhelm; Fm Mm, 3,348,964 10/1967 Good ..118/50 x Cologne-Flittard; Willi Wasser, Leverku 5en Kurt Browmzki, Opladen; Josef Primary ExaminerWilliam D. Martin Fl'ledsam Langenfeld; woligang Assistant Examiner William R. Trenor Schweicher, Leverkusen, all of Germany Anamey connouy and Hutz [73] Assignee: Agfa-Gevaert Aktiengesellschaft, Leverkusen, Germany [57] ABSTRACT [22] Filed; Feb 10 1969 A method of coating a strip of viscous solution on a movable substrate by feeding the coating solution through a narrow gap [21] Appl. No: 7 between coating apparatus which includes feed means and the movable substrate under a static pressure differential between [52] U 5 Cl 117/34 117/61 117/111 the inlet and the outlet of the gap wherein the gap is pan ofa 8/56 118/410 coating chamber of the coating apparatus. The gap is wider [51] Int Cl 3/12 (5036 N74 than the wet formed layer after formation. The coating [58] Fie'ld 61 18,50 63 chamber is connected to a vacuum means at the side of the 6 coating from which the substrate is introduced. A negative pressure is effectuated at the inlet side of the coating apparatus The vacuum means having a vacuum which seals the [56] References Cited inlet end of the narrow gap in the coating apparatus by UNITED STATES PATENTS sucking air from the vacuum means through a diaphragm which withdrawal is at a uniform rate of flow. 3,507,370 3/1970 lshiwata ..117/34 3,502,494 3/1970 lshiwata ..1 18/50 1 Claim, 1 Drawing Figure PATENTEDHAY 16 1972 3,663,292

INVENTOR? 2 PROCESS FOR COATING STRIP-FORM SUBSTRATES Reference is made to the copending US. Pat. applications Ser. No. 798,118 filed Feb. 10, 1969 and Ser. No. 797,808 filed Feb. 10, 1969. Specific reference is made to the FIGS. 1 and 2 of Application Ser. No. 797,808 and the specific description of these figures.

This invention relates to a process for coating strip-form substrates with viscous solutions preferably with photographic emulsions, by a coater the inlet end of which is sealed by a blocking vacuum chamber by sucking air from said chamber by a sucking means.

The blocking vacuum chamber screens off the emulsion chamber under negative pressure against atmospheric pressure or any other outside pressure at the inlet gap for the stripform substrate. The level of the blocking vacuum in the negative pressure chamber does not have any influence on layer formation and may be varied within wide limits as required.

However, one disadvantage affecting the use of the blocking vacuum is that any change in the blocking vacuum, for example any pressure fluctuation in the blocking vacuum, is communicated to layer formation in the form of a fluctuation in layer thickness without, however, affecting the average thickness. These troublesome fluctuations in the blocking vacuum are almost unavoidable in cases where a fan is used to produce the negative pressure, as is normally the case, due to the fact that, as a rule, not all the vanes or blades of a fan make an equal contribution towards the delivery efficiency of the fan with the result that a negative pressure generated in this way imitates the rotation frequency of the fan in the form of a superimposed alternating pressure of the same frequency.

It has now been found that these unfavorable fluctuations in the blocking vacuum which are caused by the evacuation means, can be completely avoided in accordance with the invention by passing the air sucked from the vacuum chamber by means of a sucking means, through a diaphragm arranged in the vacuum pipe leading to the sucking means, the dimensions of the diaphragm promoting a pressure jump in a ratio of at least 2:] in the direction of flow of the air drawn off under suction.

This is because the rate of flow of a gas increases in known manner with increasing pressure jump at a diaphragm, until at the critical pressure ratio, which is about 2:1 the rate of flow has become equal to the speed of sound. Thereinafter any further increases in the pressure jump does not produce any further increase in flow rate. This explains why, in our arrangement, the pressure in the evacuating means, for example a water-ring pump, may be varied as required, i.e. with any amplitude and any frequency, without any change in the rate of flow in the diaphragm and hence in the level of the blocking vacuum, providing the pressure ratio at the diaphragm remains higher than or equal to the critical ratio.

Another way of explaining the mode of operation of the present invention is as follows: changes in pressure in the evacuating means have to be equated with sound vibrations. These fluctuations in pressure or sound vibrations cannot be propagated against the sonic velocity or supersonic velocity of the air removed under suction in the diaphragm.

Normal values of the blocking vacuum are for example about 400 mm. of water, corresponding to an absolute pressure of about 9,600 mm. water column. Accordingly, to induce a rate of flow equal to the speed of sound, an absolute pressure of 4,800 mm. of water must exist at the pump end of the diaphragm. A suitable device for generating this required negative pressure, in view of the quantities of air which accumulate, is a conventional water-ring pump.

An embodiment of the invention is shown by way of example in the accompanying drawing.

A strip 1 travels over a roller 2 and is coated with a layer 4 in a coating block 3. A outlet gap 5 of an emulsion chamber 6 is several times wider than the coated layer 4 is thick. A closure 7 or the coater forms an inlet gap 8 in conjunction with the strip 1. The inlet gap 8 is sealed off by a blocking vacuum chamber 9 in which prevails a higher pressure than in the emulsion chamber 6. Walls 10 and 11 form a lock chamber to the atmosphere. A vacuum pipe 14 leading from the blocking vacuum chamber 9 to the evacuation pump 13 contains an adjustable diaphragm 15.

What is claimed is:

1. In the process of coating strip-form substrates with a coating of photographic emulsion of uniform thickness and of relatively low viscosity, the substrate being wetted in a sealed chamber in which the photographic emulsion is confined to a gap which is several times wider than thickness of the coating on the foil feeding the coating solution from the sealed chamber to the gap which is between the moving substrate and an adjacent surface of the chamber, wherein atmospheric pressure is separated from the wetting of the substrate in the sealed chamber by producing a negative pressure in a chamber immediately adjacent the sealed chamber through evacuation of air from the negative pressure chamber the improvement which consists of removing the air by suction through a passage of relatively large cross-sectional dimension and removing the air through a constriction of a relatively lesser cross-sectional dimension positioned in the passage and creating a pressure differential of at least 2 to 1 in the direction of flow of the air drawn off under suction so as to provide a rate of flow approximately equal to the speed of sound and a uniform rate of flow.

- Disclaimer 3,663,292.Peter Herzhofi, Leverkusen, Hans Gwef, Cologne-Stammheim, Fy-z'tz Moms, Cologne-Flittard, Willi Wasser, Leverkusen, Km-t Browatzlai, Opladen, Josef Ffiedsam, Lan enfeld, and Wolfgang Schwez'alwr, Leverkusen, Germany. PROCE S FOR COATING STRIP-FORM SUBSTRATES. Patent dated May 16, 1972. Disclaimer filed May 10, 1972, by the assignee, Agfa-Gevaem Aktz'engesellsckaft. Hereby f the patent subsequent to disclaims all that portion of the term 0 Feb. 29, 1989.

[Ofi'ioz'al Gazette August 29, 1972.] 

